The present invention generally relates to light-emitting semiconductor devices and more particularly to a light-emitting semiconductor device including a laser diode that produces visible optical radiation in the wavelength band of red color.
The system of AlGaInP is a III-V material having a direct-transition type band structure and provides the bandgap of as much as about 2.3 eV (540 nm in terms of optical wavelength), which is the largest bandgap value of the III-V material system except for the system of AlGaInN or the III-V material that contains B as the group III element. Thus, AlGaInP has been a target of intensive investigation in relation to high-power light emitting diode that produces visible optical radiation of green or red color. Such a high-power light emitting diodes of green to red color wavelength band has its application in color display devices. Further, the system of AlGaInP has been studied in relation to visible-wavelength laser diode for use in laser printers or in optical recording of information, such as compact disk players or DVD players.
In the laser diode designed for producing red color wavelength radiation, it has been practiced to use a material system that achieves a lattice matching with respect to a GaAs substrate. In the art of high-density recording of information, in particular, there is a demand of a high-power laser diode that operates stably even in a high-temperature or unregulated temperature environment.
In a laser diode, laser oscillation is caused as a result of stimulated emission taking place in an active layer of the laser diode, and as a result of stimulated emission, an optical beam is produced in the active layer. In order to achieve such a laser oscillation efficiently, it is necessary to confine the carriers and further the optical radiation thus produced in the active layer effectively, and for this purpose, a cladding layer having a larger bandgap energy than the active layer is provided in such a manner that the cladding layer is disposed adjacent to the active layer.
In an ordinary laser diode having a double-heterostructure, it has been practiced to use AlGaInP containing Al for the active layer in order to reduce the wavelength of the produced optical beam to the visible wavelength band. It should be noted that Al thus added has an effect of increasing the bandgap energy of the active layer.
On the other hand, Al is a very reactive element and easily forms a deep impurity level in the active layer by reacting with oxygen, which may exist in the atmosphere used for growing the III-V epitaxial layer(s) constituting the laser diode. Further, such oxygen impurity may also be contained in the source material of the III-V crystal, although with a trace amount.
In view of the problems noted above, it is preferable to reduce the Al content in the epitaxial layer constituting the active layer of the laser diode as much as possible. Thus, there is a proposal to use a GaInP quantum well for the active layer and sandwich the foregoing GaInP active layer vertically by a pair of optical waveguide layers of AlGaInP. Such a laser structure is called SCH-QW (separate confinement heterostructure quantum well) structure. Further, in relation to the SCH-QW laser diode, there is a proposal, as in the Japanese Laid-Open Patent Publication 6-77592, to apply a strain to such a quantum well layer constituting the active layer of the laser diode so as to decrease the threshold of laser oscillation further. In the case of such a laser diode using a strained quantum well structure for the active layer, the thickness of the quantum well layer is set to be smaller than a critical thickness above which lattice relaxation takes place in the active layer by creating dislocations therein. It should be noted that such a strained quantum well layer is formed by choosing a material having a lattice constant different from that of the substrate, for the quantum well layer. In the case of the visible-wavelength laser diode that oscillates at the visible wavelength such as the wavelength of 635 nm, it is indicated, in the Japanese Laid-Open Patent Publication 6-275915, that a tensile strain is more effective than a compressive strain. In the case of a quantum well layer formed on a GaAs substrate under tensile strain, the quantum well layer can take a composition of GaInP, which is closer to the GaP composition as compared with the substrate composition of GaAs. Thereby, it should be noted that the quantum well layer has an increased bandgap energy, and a quantum well layer having a suitable thickness can be used for the strained quantum well layer. In such a construction in which a sufficient thickness is secured for the quantum well layer, the adversary effect of interface defects is successfully avoided. On the other hand, the laser diode that uses the quantum well layer under tensile strain for the active layer operates in the TM-mode, and the optical beam produced by such a laser diode has a plane of polarization which is 90xc2x0 rotated as compared with the case of usual laser diode operating in the TE-mode.
As noted before, the SCH-QW construction, which uses an optical waveguide layer typically having a composition of (AlxGa1xe2x88x92x)0.5In0.5P, successfully achieves a desired optical confinement in the optical waveguide layer. On the other hand, such an optical waveguide layer, containing a large amount of Al (xxcx9c0.5) therein, has a drawback in that a damaging is tend to be caused at the free edge surface of the laser cavity as a result of recombination of the carriers associated with the Al-induced defects contained in the optical waveguide layer. Thus, such a laser diode has a drawback in that high-power operation is difficult. Further, such a laser diode has a drawback in that the reliability is degraded substantially when operated for a long period of time.
Further, such a SCH-QW has a drawback, particularly in the case it contains a heterostructure of the AlGaInP system, in that the band offset is small in the side of the conduction band. More specifically, such a structure is characterized by a small band discontinuity (xcex94Ec) in the conduction band between the active layer and the cladding layer, and the carriers (electrons in particular) injected into the active layer from the cladding layer easily cause an overflow. Thereby, the laser diode shows a heavy temperature dependence in the laser threshold characteristic, particularly the threshold current, and expensive temperature regulation has been needed for stable operation thereof. This problem of poor temperature characteristic of the laser oscillation becomes rapidly worse with decreasing oscillation wavelength. For example, the temperature dependence of the laser oscillation characteristic for the laser diode operating at the wavelength of 635 nm is much worse than that of the laser diode operating at the wavelength of 650 nm.
In order to overcome the foregoing problem of temperature dependence of the laser diode, there is a proposal, as in the Japanese Laid-Open Patent Publication 4-114486, to enhance the carrier confinement efficiency by providing a multiple quantum barrier (MQB) structure between the active layer and the cladding layer, wherein the MQB structure includes a stacking of a number of extremely thin layers. This proposal, however, is turned out to be not realistic because of its structural complexity and the difficulty of thickness control of each thin layer of the MQB structure. In order to obtain the desired effect according to this conventional approach, it is necessary to form the individual layers of the MQB structure to be flat with the precision of atomic layers.
Thus, there has been a distinct limitation in the improvement of temperature dependence and further in the decrease of laser oscillation wavelength, as long as the laser diode is constructed on a conventional GaAs substrate. More specifically, it has been not possible to realize a laser diode, according to such a conventional approach, that operates at the wavelength of 635 nm in the operational environment of 80xc2x0 C., with the output optical power of 30 mW or more over a long duration such as ten thousand hours or more.
In view of the foregoing problems associated with the use of the GaAs substrate, there is a proposal to construct a laser diode on a GaP substrate. A GaP substrate has a smaller lattice constant as compared with a GaAs substrate and is thought more appropriate for the substrate for growing thereon epitaxial layers of the AlGaInP system. Thus, there is a proposal in the Japanese Laid-Open Patent Publication 6-53602 of a laser diode constructed on a GaP substrate, wherein the laser diode includes a cladding layer of AlGaP having a composition of AlyGa1xe2x88x92yP (0xe2x89xa6yxe2x89xa61) and a compressed active layer of GaInP having a composition of GaxIn1xe2x88x92xP (0 less than x less than 1), which is a material having a direct-transition type band structure. In the proposed device, the active layer is doped with N forming an isoelectronic trap. This prior art device, while being able to decrease the oscillation wavelength of the laser diode and simultaneously reduce the amount of Al contained in the active layer, still has a drawback in that there remains a lattice misfit of as much as 2.3% with respect to the GaP substrate, even in the case the active layer has a composition of Ga0.7In0.3P. It should be noted that the GaInP mixed crystal maintains the direct-transition type band structure and has a lattice constant closest to the lattice constant of the GaP substrate at the foregoing composition of Ga0.7In0.3P. The existence of the foregoing lattice misfit is not preferable as such a lattice misfit reduces the critical thickness of the active layer. In order to avoid the creation of the lattice misfit dislocations in such a system, it is necessary to reduce the thickness of the active layer significantly, while the use of such an extremely small thickness for the active layer is not practical.
Further, there is a proposal in the Japanese Laid-Open Patent Publication 5-41560 in which a double heterostructure including an AlGaInP active layer and an AlGaInP cladding layer is formed on a GaAs substrate with an intervening buffer layer of GaPAs, wherein the AlGaInP layers constituting the double heterostructure have a composition of (AlGa)aIn1xe2x88x92aP (0.51 less than axe2x89xa60.73) and a corresponding lattice constant intermediate to the lattice constant of GaAs and the lattice constant of GaP. The buffer layer has a composition represented as GaPxAs1xe2x88x92x and eliminates the lattice misfit between the GaAs substrate and the double heterostructure.
FIG. 1 shows the relationship between the bandgap energy and composition in the AlGaInP system used in the foregoing prior art, wherein the continuous lines of FIG.1 represent the composition that provides the direct-transition type band structure, while the broken lines represent the composition that provides the indirect-transition type band structure.
Referring to FIG. 1, the foregoing composition of AlGaInP ((AlGa)aIn1xe2x88x92aP; 0.51 less than axe2x89xa60.73) having the intermediate lattice constant between GaAs and GaP falls in the region defined by the composition AlInP and the composition GaInP, wherein the AlInP composition is an intermediate composition on the line connecting the AlP composition and the InP composition. Further, the GaInP composition is an intermediate composition on the line connecting the GaP composition and the InP composition.
According to the foregoing approach of the Japanese Laid-Open Patent Publication 5-41560, it is possible to use the AlGaInP composition characterized by a lager bandgap as compared with the material system that achieves a lattice matching with the GaAs substrate for the cladding layer or active layer of the laser diode. Thus, the foregoing prior art is advantageous for realizing a laser diode operating at the visible wavelength of 600 nm or shorter. This wavelength band corresponds to green to yellow color radiation.
On the other hand, the foregoing prior art structure is not suitable for the laser diode operable at the wavelength of 635 nm or 650 nm corresponding to red color radiation. For example, it can be seen from FIG. 1 that the bandgap energy changes with lattice constant in the system of GaInP with a steeper ratio as compared with the system of AlInP up to the composition in which the Ga content is 0.73, as represented in FIG. 1 by a continuous line. Thus, when the composition of the GaInP active layer is tuned to the wavelength of 635 nm, the lattice constant of the active layer takes a value close to that of the GaAs substrate. On the other hand, it is preferable to increase the bandgap energy of the AlInP cladding layer as much as possible for effective confinement of the carriers. In order to do this, it is preferable to choose the composition close to AlP for the cladding layer. However, such a selection of the cladding layer composition invites a heavy accumulation of compressive strain in the active layer.
Accordingly, it is a general object of the present invention to provide a novel and useful light-emitting semiconductor device wherein the foregoing problems are eliminated.
Another and more specific object of the present invention is to provide a light-emitting laser diode including a laser diode operable in a high-temperature environment at the visible wavelength band of red such as 635 nm or 650 nm.
Another object of the present invention is to provide an efficient light-emitting semiconductor device, including laser diode and light-emitting diode, operable in the room temperature environment at the visible wavelength band of 600 nm or shorter.
Another object of the present invention is to provide a light-emitting semiconductor device, comprising:
a semiconductor substrate;
an active layer provided above said semiconductor substrate, said active layer producing a red optical radiation; and
a cladding layer provided above said semiconductor substrate adjacent to said active layer,
said active layer comprising a III-V material in the system of AlGaInPAs having a composition represented as (AlxGa1xe2x88x92x)xc3xa1In1xe2x88x92xc3xa1PtAs1xe2x88x92t (0x less than 1, 0 less than xcex11, 0 t 1),
said cladding layer containing Al and comprising a III-V material in the system of AlGaInPAs having a composition represented as (AlyGa1xe2x88x92y)xc3xa1In1xe2x88x92xcex2PVAs1xe2x88x92V (0 less than y 1, 0.5 less than xcex21, 0 less than V 1), said cladding layer having a bandgap larger than a bandgap of said active layer and a lattice constant intermediate between a lattice constant of GaP and a lattice constant of GaAs.
Another object of the present invention is to provide a light-emitting semiconductor device, comprising:
a semiconductor substrate;
an active layer provided above said semiconductor substrate, said active layer producing a red optical radiation;
a cladding layer provided above said semiconductor substrate adjacent to said active layer; and
an optical waveguide layer interposed between said active layer and said cladding layer,
said active layer comprising a single quantum well layer of a III-V material in the system of AlGaInPAs having a composition represented as (AlxGa1xe2x88x92x)xc3xa1In1xe2x88x92xc3xa1PtAs1xe2x88x92t (0xe2x89xa6x less than 1, 0 less than xcex1xe2x89xa61, 0xe2x89xa6txe2x89xa61),
said cladding layer containing Al and comprising a III-V material in the system of AlGaInPAs having a composition represented as (AlyGa1xe2x88x92y)xcex2In1xe2x88x92xcex2PvAs1xe2x88x92V (0 less than yxe2x89xa61, 0.5 less than xcex2xe2x89xa61, 0 less than vxe2x89xa61), said cladding layer having a bandgap larger than a bandgap of said active layer and a lattice constant intermediate between a lattice constant of GaP and a lattice constant of GaAs,
said optical waveguide layer comprising a III-V material in the system of AlGaInPAs having a composition represented as (AlzGa1xe2x88x92z)xc3xa1In1xe2x88x92xcex3PuAs1xe2x88x92u (0xe2x89xa6z less than 1, 0.5 less than xcex3xe2x89xa61, 0 less than uxe2x89xa61), said optical waveguide layer having a bandgap larger than said bandgap of said active layer but smaller than said bandgap of said cladding layer.
Another object of the present invention is to provide a light-emitting semiconductor device, comprising:
a semiconductor substrate;
an active layer provided above said semiconductor substrate, said active layer producing a red optical radiation;
a cladding layer provided above said semiconductor substrate adjacent to said active layer; and
an optical waveguide layer interposed between said active layer and said cladding layer,
said active layer having a multiple quantum well structure comprising a plurality of quantum well layers of a III-V material in the system of AlGaInPAs having a composition represented as (Alx1Ga1xe2x88x92x1)xcex11In1xe2x88x92xcex11Pt1As1xe2x88x92t1 (0xe2x89xa6x1 less than 1, 0 less than xcex11xe2x89xa61, 0xe2x89xa6t1xe2x89xa61) and a plurality of barrier layers of a III-V material in the system of AlGaInPAs having a composition represented as (Alx2Ga1xe2x88x92x2)xcex12In1xe2x88x92xcex12Pt2As1xe2x88x92t2 (0xe2x89xa6x2 less than 1, 0 less than xcex12 less than 1, 0xe2x89xa6t2xe2x89xa61), each of said barrier layers having a bandgap larger than a bandgap of said quantum well layer,
said cladding layer containing Al and comprising a III-V material in the system of AlGaInPAs having a composition represented as (AlyGa1xe2x88x92y)xcex2In1xe2x88x92xcex2PvAs1xe2x88x92v (0 less than yxe2x89xa61, 0.5 less than xcex21, 0 less than vxe2x89xa61), said cladding layer having a bandgap larger than a bandgap of said quantum well layer in said active layer and a lattice constant intermediate between a lattice constant of GaP and a lattice constant of GaAs,
said optical waveguide layer comprising a III-V material in the system of AlGaInPAs having a composition represented as (AlzGa1xe2x88x92z)xcex3In1xe2x88x92xcex3PuAs1xe2x88x92u (0xe2x89xa6z less than 1, 0.5 less than xcex3xe2x89xa61, 0 less than uxe2x89xa61), said optical waveguide layer having a bandgap larger than said bandgap of said quantum well layer in said active layer but smaller than said bandgap of said cladding layer.
According to the present invention, an efficient laser oscillation is obtained at the visible, red optical wavelength by using a cladding layer having a lattice constant intermediate between GaAs and GaP. It should be noted that the system of AlGaInP having such a lattice constant can provide a large bandgap effective for carrier confinement while reducing the Al-content therein. Further, the present invention enables of using an Al-free composition for the optical waveguide layers in producing a red wavelength beam. Thereby, the laser diode can be operated at a high output power without causing damage on the optical cavity edge surface. By introducing As into the cladding layer, it becomes possible to suppress the hillock formation.
Another object of the present invention is to provide a laser diode, comprising:
a semiconductor substrate; a first cladding layer of AlGaInP provided above said semiconductor substrate, said first cladding layer having a first conductivity type and a lattice constant intermediate between a lattice constant of GaAs and a lattice constant of GaP;
an active layer of GaInPAs provided above said first cladding layer;
a second cladding layer of AlGaInP provided above said active layer, said second cladding layer having second conductivity type and a lattice constant substantially identical with said lattice constant of said first cladding layer;
an etching stopper layer of GaInP provided above said second cladding layer, said etching stopper layer having said second conductivity type;
a third cladding layer of AlGaInP provided above said etching stopper layer, said third cladding layer having said second conductivity type and a lattice constant substantially identical with said lattice constant of said first cladding layer;
said etching stopper layer having a lattice constant generally equal to said lattice constant of said first cladding layer and a bandgap substantially larger than a bandgap of said active layer.
Another object of the present invention is to provide a method of fabricating a laser diode, comprising the steps of:
forming a layered structure, on a semiconductor substrate, such that said layered structure includes a first cladding layer of AlGaInP provided above said semiconductor substrate, said first cladding layer having a first conductivity type and a lattice constant intermediate between a lattice constant of GaAs and a lattice constant of GaP; an active layer of GaInPAs provided above said first cladding layer; a second cladding layer of AlGaInP provided above said active layer, said second cladding layer having a second conductivity type and a lattice constant substantially identical with said lattice constant of said first cladding layer; an etching stopper layer of GaInP provided above said second cladding layer, said etching stopper layer having said second conductivity type; a third cladding layer of AlGaInP provided above said etching stopper layer, said third cladding layer having said second conductivity type and a lattice constant substantially identical with said lattice constant of said first cladding layer; said etching stopper layer having a lattice constant generally equal to said lattice constant of said first cladding layer and a bandgap substantially larger than a bandgap of said active layer,
etching said third cladding layer to form a stripe region while using said etching stopper layer as an etching stopper, until said etching stopper layer is exposed at both lateral sides of said stripe region; and
filling a current confinement region on said exposed etching stopper layer at both lateral sides of said stripe region.
Another object of the present invention is to provide a method of fabricating a laser diode, comprising the steps of:
forming a layered structure, on a semiconductor substrate, such that said layered structure includes a first cladding layer of AlGaInP provided above said semiconductor substrate, said first cladding layer having a first conductivity type and a lattice constant intermediate between a lattice constant of GaAs and a lattice constant of GaP; an active layer of GaInPAs provided above said first cladding layer; a second cladding layer of AlGaInP provided above said active layer, said second cladding layer having a second conductivity type and a lattice constant substantially identical with said lattice constant of said first cladding layer; an etching stopper layer of GaInP provided above said second cladding layer, said etching stopper layer having said second conductivity type and a lattice constant generally equal to said lattice constant of said first cladding layer, said etching stopper layer having a bandgap substantially larger than a bandgap of said active layer; and a current-confinement layer provided above said etching stopper layer, said current-confinement layer having said first conductivity type;
etching said current confinement layer to form a stripe opening while using said etching stopper layer as an etching stopper, until said etching stopper layer is exposed along said stripe opening; and
depositing a third cladding layer of AlGaInP having said second conductivity type on said current-confinement layer so as to fill said stripe opening.
According to the present invention, it is possible to form a current-confinement structure by a wet etching process by using an etching stopper of GaInP while avoiding unwanted optical radiation in the wavelength of red optical radiation by sun an etching stopper layer.
Another object of the present invention is to provide a light-emitting semiconductor device, comprising:
a semiconductor substrate;
a first cladding layer of n-type AlGaInP provided above said semiconductor substrate, said first cladding layer having a composition represented as (Alx1Ga1xe2x88x92x1)y1In1xe2x88x92y1P (0xe2x89xa6x1, 0.51 less than y1xe2x89xa61) and a lattice constant intermediate between a lattice constant of GaAs and a lattice constant of GaP;
an active layer provided above said first cladding layer;
a second cladding layer of p-type AlGaInP provided above said active layer, said second cladding layer having a composition substantially identical with said composition of said first cladding layer;
wherein a multiple quantum barrier structure is interposed between said active layer and said second cladding layer,
said multiple quantum barrier structure comprising an alternate repetition of a quantum well layer having a composition represented as (Alx2Ga1xe2x88x92x2)y2In1xe2x88x92y2P (0xe2x89xa6x2xe2x89xa61, 0xe2x89xa6y1xe2x89xa61) and a bandgap smaller than a bandgap of said second cladding layer, and a barrier layer having a composition substantially identical with said composition of said second cladding layer.
Another object of the present invention is to provide a light-emitting semiconductor device, comprising:
a semiconductor substrate;
a first cladding layer of n-type AlGaInP provided above said semiconductor substrate, said first cladding layer having a composition represented as (Alx1Ga1xe2x88x92x1)y1In1xe2x88x92y1P (0xe2x89xa6x1xe2x89xa61, 0.51 less than y1xe2x89xa61) and a lattice constant intermediate between a lattice constant of GaAs and a lattice constant of GaP;
an active layer provided above said first cladding layer;
a second cladding layer of p-type AlGaInP provided above said active layer, said second cladding layer having a composition substantially identical with said composition of said first cladding layer;
wherein a carrier blocking layer is interposed at least between said active layer and said second cladding layer, said carrier blocking layer having a composition represented as (Alx3Ga1xe2x88x92x3)y1In1xe2x88x92y1P (0xe2x89xa6x1xe2x89xa6x3xe2x89xa61, 0.51 less than y1xe2x89xa61) and a bandgap larger than a bandgap of said second cladding layer, said carrier blocking layer having a lattice constant generally matching with a lattice constant of said second cladding layer.
Another object of the present invention light-emitting semiconductor device, comprising:
a semiconductor substrate;
a first cladding layer of n-type AlGaInP provided above said semiconductor substrate, said first cladding layer having a composition represented as (Alx1Ga1xe2x88x92x1)y1In1xe2x88x92y1P (0xe2x89xa6x1xe2x89xa61, 0.51 less than y1xe2x89xa61) and a lattice constant intermediate between a lattice constant of GaAs and a lattice constant of GaP;
an active layer provided above said first cladding layer;
a second cladding layer of p-type AlGaInP provided above said active layer, said second cladding layer having a composition substantially identical with said composition of said first cladding layer;
wherein a carrier blocking layer is interposed at least between said active layer and said second cladding layer, said carrier blocking layer having a composition represented as (Alx4Ga1xe2x88x92x4)y4In1xe2x88x92y4P (0 x4 1, 0.51 less than y1 less than y4 1) and a bandgap larger than a bandgap of said second cladding layer, said carrier blocking layer having a lattice constant smaller than a lattice constant of said second cladding layer.
According to the present invention, an efficient light-emitting semiconductor device producing red color radiation is obtained by using a carrier blocking layer that blocks the carriers injected into the active layer from causing overflowing.
Other objects and further features of the present invention will become apparent from the following detailed description when red in conjunction with the attached drawings.